Hardness is how resistant something is to being scratched, dented, or shaped. A diamond has tremendous hardness because almost nothing can scratch it. Steel has more hardness than aluminum, which is why steel tools can cut and shape aluminum but not the other way around. Your fingernail has enough hardness to scratch soft wood but not enough to scratch glass.

Scientists measure hardness using specific scales. The Mohs scale ranks minerals from 1 to 10, with talc at 1 (so soft you can scratch it with your fingernail) and diamond at 10 (the hardest natural substance). Engineers testing metals might use different methods, pressing a hard point into the material to see how deep the dent goes.

Hardness matters in countless ways. When choosing materials for a skateboard deck, you need wood with the right hardness: too soft and it breaks easily, too hard and it becomes brittle. A sculptor selects marble over granite partly because marble's lower hardness makes it easier to carve. The hardness of a pencil's graphite determines whether it makes light, crisp lines or dark, smudgy ones.

The word can also describe difficulty or severity: you might describe the hardness of a challenging math problem or the hardness of a winter. But in its physical sense, hardness is simply about resistance to being scratched, dented, or deformed.